This study is designed to investigate the nature and pathogenic significance of the increased frequency of HLA-B8 observed in juvenile dermatomyosis (JDMS). One hundred patients with juvenile onset of JDMS as defined by the presence of rash; elevated CPK, aldolase, LDH, or SGOT; abnormal electromyogram; and muscle biopsy will be obtained from four major centers. Thirty children and thirty adults with polymyositis will also be studied. All the probands will be interviewed by the study nurse, the medical records reviewed and family and personal data concerning malignancy and auto-immune disease recorded. Blood will be drawn for HLA -A, -B and -C locus typing and for determination of the following auto-antibodies: smooth muscle, striated muscle, rheumatoid factor, mitochondrial, nuclear, thyroglobulin, thyroid microsomal, and gastric parietal cells. In addition, ANA antibody and its sub-components, including PM-1 specific for myositis, will be determined. The families of fifteen local patients and of age, sex and race matched controls will be intensively studied. The following will be determined: HLA-A, -B, -C, -D and "Ia" antigens, auto-antibody and family history of auto-immune or malignant disease. Our findings will be analyzed for an association between B8 and auto-immunity and the relationship of both of these findings to JDMS will be examined.